1. Field of the Invention
The present invention relates to a servo system for positioning a read/write head over a destination track of a storage medium and, in particular, to a servo system which is used for a hard disc drive.
2. Description of the Related Art
In a conventional hard disk drive (HDD), a head is moved to a destination track (destination cylinder) of a storage medium, such as a disk, to gain access to data on that destination track. The servo system positions a head over the destination track of the disk. The servo system detects a head's current position (current track) of the disk through the utilization of initially recorded servo data and implements a head positioning control operation based on the head's current position. The servo data is recorded on a dedicated servo surface. Two types of heads are employed: one type being a single servo head corresponding to the servo surface and the other type being a plurality of data heads corresponding to a data surface. The servo head reads servo data from the servo surface and delivers it to a position transducer, that is a position signal generator. The position transducer delivers a position signal corresponding to the current position of the head. The servo head and data heads are mounted on a carriage and moved in a radial direction of the disk by driving the carriage. The respective data head follows the servo head under head positioning control by the servo system and is moved to the destination track of the disk.
The servo data is composed of position data which is differently varied in magnetic flux in accordance with a sync signal and track position. The sync signal is composed of a sync clock and sync data and circumferentially recorded at equal intervals on the servo track of the servo surface. Further, the sync signal is continuously recorded radially in the same position of all the servo tracks on the servo surface.
The servo circuit separates a sync signal and position data from servo data read out by the servo head. The servo circuit includes a position transducer for transforming the position data to a position signal and velocity detector for differentiating the position signal and outputting a velocity signal corresponding to an actual speed of the head. This technique is disclosed, for example, in U.S. Pat. No. 4,238,809.
In the servo system, if servo data is normally not recorded on the servo surface of the disk, it is naturally not possible to position the head under head positioning control. In the even of a malfunction relating to the servo data, therefore, it is necessary to detect the malfunction. A servo data malfunction detection system is conceived for monitoring the level of servo data read by a servo head and delivering a malfunction detection signal when that level falls below a preset reference level. Even in the case where servo data is normally recorded on the disk, a servo data malfunction signal is delivered as an output signal when the output signal of the servo head wholly goes low. Stated in another way, in the event of a malfunction on a servo data reading system including a servo head, a servo data malfunction detection operation is done even if servo data is recorded as normal data.